A common way to force a tunable laser gain medium to produce output at a specific wavelength is to couple the gain medium to an external laser cavity. The external cavity disperses the intracavity radiation into its spectral components and selects the wavelength whose oscillation in the laser cavity is sustained. Typically, this wavelength has the highest gain/loss ratio among the competing wavelengths (cavity modes).
External cavity tuning is generally the preferred way of laser tuning when wide tuning range is required. Besides wide range, it provides wavelength reproducibility and stability over time. High tuning speed, however, has been difficult to achieve simultaneously with a wide tuning range, with external cavity designs in the past.
FIG. 1a illustrates the traditional Litrow design of an external cavity 10, with gain element 12, lens 14 and diffraction grating 16. In this case, the laser power is out coupled from the cavity through the zero-th order reflection of grating 16 [e.g., Maulini, 2006]. Laser tuning is achieved by grating rotation. In this simplest form of the Litrow design, tuning by grating rotation has an additional disadvantage that it changes the direction of the output beam. Laser tuning by mechanical rotation of the grating, although amenable to computer control [e.g., Ignjatijevic and Vujkovic-Cvijin et al., 1985], is clearly not well-suited for fast laser tuning.
An approach which makes external cavity tuning both fast and digitally programmable, while retaining the advantages of external cavity tuning, makes use of a spatial light modulator (SLM) located inside the cavity, to spatially modulate the reflectivity of one of the “cavity mirrors.” With a dispersive device inside the cavity, the spatial modulation of the SLM translates into spectral modulation, resulting in lasing action at the desired wavelength. A device which uses a SLM for external cavity tuning has been described previously by Gutin [Gutin, 2001, Gutin, 2003] and by Breede et al. [Breede et al., 2007].
In a typical example of such a cavity 20 shown on FIG. 1b, the laser gain element 22 is anti-reflection (AR) coated at the facet facing the external cavity, while the coating of the opposite facet is suitable for laser beam out coupling. The light emerging from the AR coated facet is collimated with a high numerical aperture (NA) lens 24 and directed to a diffraction grating or other dispersive element 26. The first-order diffracted beam is captured by a lens (not shown) and directed towards the SLM 28, typically represented by a Digital Micromirror Array (DMA) [Hornbeck, 1991, Digital Light Innovations, 2011]. Dispersed beams of different wavelengths are spatially resolved on the surface of the DMA where individually addressable micromirrors select the wavelength which is sent back to oscillate in the cavity. The DMA operates by tilting its micromirrors between two predetermined stable states, under digital electronic control. In one of the states, the micromirrors reflect the beam back to the gain element (the “on” state), while in the other state, the light is decoupled from the cavity (the “off” state). The wavelength corresponding to the spatial position on the DMA with micromirrors turned “on” will oscillate in the cavity. As a consequence, laser tuning under fast digital electronic control inherent to the DMA modulator becomes possible. Since the digitally controlled external cavity laser has the ability to turn on or off any wavelength in any order, any laser wavelength can be accessed at random, as opposed to sequential wavelength tuning. Random access tuning is a unique feature of digitally controlled external cavity laser tuning.